1. Technical Field
The present invention pertains to signal synthesizers. In particular, the present invention pertains to a digital synthesizer that generates a high frequency signal from a plurality of intermediate frequency waveforms.
2. Discussion of Related Art
Digital communication transmitters operate by modulating a carrier waveform with digital data. One possible way to generate carrier waveforms in digital systems is by utilizing a direct digital synthesizer (DDS). The DDS may be implemented by a field programmable gate array (FPGA), an application specific integrated circuit (ASIC) or other logic device and includes a phase accumulator, a phase-to-amplitude converter and a digital-to-analog converter (DAC). The phase accumulator is typically an accumulator that increments a phase value by a phase increment each time interval. The phase increment is determined by the sampling frequency and carrier frequency. The phase-to-amplitude converter is generally implemented by a sine look-up table read-only memory (ROM) that receives phase information from the phase accumulator and produces digital amplitude values. The DAC converts the digital amplitude values to an analog waveform. A conventional DDS generates a sine wave or carrier waveform. Phase, frequency and/or amplitude modulation may be performed in the digital domain within the DDS and/or within the analog domain subsequent to the digital to analog conversion.
The Nyquist rate defines the minimum sampling rate needed to generate a digital carrier waveform as twice the frequency of that waveform. Generally, realistic applications require a sampling rate greater than twice the carrier frequency to produce a viable signal. With the high frequencies required for most digital communications applications (e.g., approximately 800 MHz for cellular applications, approximately 1500 MHz for a Global Positioning System (GPS) L1 type signal, etc.), it is impractical and inefficient to employ an ASIC, FPGA or other logic device operating at a sampling frequency of more than twice the frequency of the carrier waveform.